Screen printing is older than the electronics industry, and was initially developed in the first decade of this century out of the even older art of printing with stencils. In a modern screen printing process, a stainless steel screen is positioned on a ceramic or glass substrate, the screen having portions masked such that unmasked areas represent the areas to be printed. A printing ink containing a powder of solid particles is then placed on the screen, and a squeegee is drawn over the screen to force the ink through the unmasked areas of the screen. The screen is then withdrawn, and the ink-coated substrate is fired, leaving a pattern of solid material (e.g., a conductor, dielectric or resistor) on the substrate.
A typical screen printing process may generally be described with reference to FIGS. 1A-1C. In FIG. 1A, reference numeral 12 represents the substrate on which the printing occurs. The printing apparatus includes screen 14 and squeegee 24. Screen 14 includes frame 16 and mesh 18, the mesh comprising a square array of filaments 20. In the cross-sectional view in FIGS. 1A-1C, only those filaments extending into and out of the plane of the drawing are shown. In operation, a quantity of ink 30 is placed on screen 14 at one side of mesh 18, and the squeegee is then drawn across the mesh in a print stroke, as illustrated in FIGS. 1B and 1C. During the print stroke, right hand surface 26 of the squeegee pushes ink 30 across the screen, and the lowermost edge of the squeegee comprises a contact edge 28 that bears against mesh 18. The contact edge locally deforms the mesh such that the mesh is forced down into contact with substrate 12, and the squeegee forces ink 30 to pass through the mesh onto the substrate. The bead or globule of ink 30 bounded by screen 14 and surface 26 of squeegee 24 is referred to herein as the ink "before the squeegee." It should be understood that FIGS. 1A-1C are an exaggerated schematic view, and that the quantity of ink before the squeegee is much larger than the quantity of ink that passes through mesh 18 during a print stroke. Thus for most purposes, the decrease in the volume of the ink before the squeegee during a print stroke due to ink passing through the screen is negligible.
Because the unused ink is moved from one end of the screen to the other during a print stroke, an automated and continuous printing process is not possible unless the ink is returned to its starting position to begin the next print stroke. The ink is usually returned by the so-called flood stroke. During the flood stroke, the squeegee is moved a certain distance above the screen, and then moved back to its starting position to begin the next print stroke. During the flood stroke, a film of ink having a thickness approximately equal to the distance of the squeegee from the screen during the subsequent flood stroke is left on the screen. Thus, when the squeegee moves through the subsequent print stroke, the amount of ink before the squeegee increases as the flood deposit is scraped off the screen by the squeegee.
To date, screen printing has been treated as an art, without any mathematical definition or correlation of the printing parameters with printing results. In particular, printing conditions are generally established based on empirical results or operator skill or experience. Because of the unavailability of generally applicable models of the printing process, screen printing machines are typically designed to hold the squeegee speed and squeegee angle constant during a printing stroke, in an attempt to achieve consistent printing conditions. However, as described above, at least one parameter, the amount of ink before the squeegee, may change during a print stroke. The result is that printing conditions are not in fact kept constant during a print stroke, in spite of the constant machine conditions.
For many applications, the above considerations have resulted in the abandonment of the use of a continuous process for screen printing. In particular, for high accuracy applications, the flooding of the screen during the flood stroke is avoided. Instead, an operator manually picks up the ink at the end of the print stroke with a spatula, and places the ink as a bead before the squeegee at the other end of the screen. With this procedure, although cumbersome and not suitable for automation, the printing results do not change significantly during the long print strokes required for large substrates, because the squeegee moves an almost constant amount of ink during each printing stroke.